TypeErrror on Windows with test functions / model tree

Hi,

I'm getting a fairly strange `TypeError` which I can't seem to resolve. Any help on this one?

To reproduce, I start from a fairly clean install and just `pip install mayavi`, which leads to the following version of (error) packages:
```python
pyface 7.3.0
PyQt5 5.12.3
PyQt5-sip 12.9.0
traits 6.3.2
traitsui 7.2.1
````

I then proceed to start a python interpreter, and then go for
```python
from mayavi import mlab
mlab.test_flow() # Choose any test function here, all work on this call
```
So far so good. However if I open the tree view and try to add an module to the dataset underlying the example plot the interpreter crashes with the error below - independent of the module I try to add and independent of the starting point, e.g. the chosen example.

```python
Traceback (most recent call last):
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\pyface\ui\qt4\action\action_item.py", line 184, in _qt4_on_triggered
self.controller.perform(action, action_event)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\qt4\tree_editor.py", line 1189, in perform
self.ui.do_undoable(self._perform, action)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\ui.py", line 655, in do_undoable
action(*args, **kw)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\qt4\tree_editor.py", line 1192, in _perform
node, object, nid = self._data
TypeError: cannot unpack non-iterable NoneType object
```